As her hands enter your cranium and penetrate your brain to let her rewrite your mind, your former girlfriend see all your memories, including all the one you have of her. And it's quite a lot, since before you started dating, you already were childhood friend. (In fact, you both had dated separately before realising how much your feelings for each others had changed since you were children.)
It slowly restored her sanity and ability to feel, making her realise what kind of monster she had briefly become.
Alas, it was too little too late, as almost nothing of what made you, you was left, turning you into little more than a puppet for her to manipulate, like all her other slaves. She wish her new form let her cry, but is left almost unable to express her grief at your loss instead.